A device comprising an organic material is expected to be applied to wide range of elementary devices and applications such as an organic electroluminescence device (hereinafter referred to as organic EL device), an organic transistor, an organic solar cell, an organic semiconductor, etc. Other devices comprising a hole injection transport layers include a quantum-dot light emitting device, an oxide compound solar cell, etc.
The organic EL device is a charge injection type light emitting device, which uses emission of light generated by the recombination of holes and electrons that have reached a light emitting layer. This type of organic EL devices have been actively developed since, in 1987, T. W. Tang et al. demonstrated that a device obtained by stacking thin films each made of a fluorescent metal chelate complex and diamine molecules can emit high-intensity light at a low driving voltage.
The device structure of the organic EL device comprises a cathode/an organic layer/an anode. In early organic EL devices, this organic layer had a two-layered structure comprising a light emitting layer/a hole injection layer. In recent years, however, to obtain high light emission efficiency and long driving life, various kinds of multi-layered structures have been proposed, such as a five-layered structure comprising an electron injection layer/an electron transport layer/a light emitting layer/a hole transport layer/a hole injection layer.
The layers other than the light emitting layer, such as the electron injection layer, the electron transport layer, the hole transport layer and the hole injection layer, are said to be effective in allowing smooth injection and/or transport of charge into the light emitting layer, maintaining the balance between the electronic current and hole current by blocking charge, or preventing diffusion of light energy excitons, for example.
In order to improve charge transport ability and charge injection ability, it has been attempted to increase electrical conductivity, by mixing an oxidizing compound with a hole transporting material (Patent Literatures 1 and 2).
In Patent Literature 1, the following compound are used as the oxidizing compound, that is, as the electron accepting compound: a compound comprising a triphenylamine derivative and a counter anion such as antimony hexafluoride, and a compound with extremely high electron-accepting properties, in which cyano groups are bound to carbons of a carbon-carbon double bond, such as 7,7,8,8-tetracyanoquinodimethane.
In Patent Literature 2, general oxidizers are exemplified as the oxidizing dopant; moreover, metal halides, Lewis acids and organic acids, and salts of arylamines and metal halides or Lewis acids, are mentioned.
In Patent Literatures 3 to 6, metal oxides, being compound semiconductors, are used as the oxidizing compound, that is, as the electron-accepting compound. In order to obtain a hole injection layer with excellent injection properties and excellent charge transfer properties, for example, a thin film is formed by evaporation using a metal oxide such as vanadium pentoxide or molybdenum trioxide, or a mixed film is formed by co-evaporation of a molybdenum oxide and a low-molecular amine compound.
In Patent Literature 7, a production method is mentioned as an attempt to form a coating film of vanadium pentoxide, which is such a method that a solution in which oxovanadium(V)tri-i-propoxide oxide is dissolved, is used as the oxidizing compound, that is, as the electron-accepting compound, and a mixed coating film of the solution and a hole transporting polymer is formed and then hydrolyzed in water vapor to form a charge transfer complex.
In Patent Literature 8, as an attempt to form a coating film of molybdenum trioxide, it is mentioned that molybdenum trioxide is physically ground to produce fine particles and then dispersed in a solution to produce a slurry, and the slurry is applied to form a hole injection layer and thus to produce a long-life organic EL device.
The organic transistor is a thin-film transistor in which an organic semiconductor material comprising a π-conjugated, organic high-molecular compound or a π-conjugated, organic low-molecular compound is used in the channel region. A common organic transistor has a structure comprising a substrate, a gate electrode, a gate insulating layer, a source electrode, a drain electrode and an organic semiconductor layer. In the organic transistor, the gate voltage applied to the gate electrode is changed to control the charge amount of an interface between the gate insulating layer and the organic semiconductor layer, thus changing the current value between the source electrode and the drain electrode for switching.
As an attempt to increase the on-current value of the organic transistor and stabilize the device properties by reducing the charge injection barrier between the organic semiconductor layer and the source or drain electrode, is it known to increase the carrier density in the organic semiconductor layer adjacent to the electrode, by introducing a charge transfer complex into the organic semiconductor layer (for example, Patent Literature 9).
However, even though the oxidizing materials as disclosed in Patent Literatures 1 to 9 are used as the hole transporting material, it is still difficult to realize a long-life device or there is a need for a further increase in device lifetime.
In Patent Literature 10, as a technique for increasing device lifetime by forming a hole injection transport layer by a solution applying method, a device comprising a hole injection transport layer that contains a reaction product of a molybdenum or tungsten complex, is disclosed. However, there is still a demand for a further increase in device lifetime.